Hydrostatic bearings have been in use for a very long time, and recent improvements in compensator design, such as discussed in U.S. Pat. Nos. 5,164,237 and 5,281,032 provided means to allow water (or similar water-derived or related fluids herein generically referred to as `water`) to be used as a working fluid for the bearings. However, although water is an ideal fluid from a heat transfer perspective, and for reducing the shear forces on the spindle, its lower viscosity makes it more likely to cause turbulent flow. When a flow becomes turbulent, it causes greater viscous shear losses, and the power rate increase with speed can actually rise. In the past, when oil was used, flows rarely became turbulent and because of the high viscosity of oil, they still generated far too much heat to be used at high speeds which might give rise to turbulence.
Now with the use of water made possible by the above-cited patents, high laminar speeds can be obtained reasonably, and indeed, even when the flow becomes turbulent, it still only generates half the shear losses of oil. The present invention is concerned with major advancements in optimizing the shape of the bearing regions most effectively to handle high speed water flow which by its nature and the nature of wanting to run spindles fast, creates a need to run the bearings in a turbulent state. As will also later be shown, the designs for turbulence, also make the bearing more robust and more accurate even when they are run with oil or in the non turbulent state.
The present invention allows designers to increase the allowable speed and accuracy of rotary motion hydrostatic bearings. Design methodologies are presented for designing self compensating hydrostatic bearing spindle pockets and compensators to minimize the effects variation in stiffness with angular position, cavitation in bearing pockets, and turbulent shear power generation. Although it may seem that these are unrelated effects, the design solutions require substantial overlap and are thus presented in combination for optimal results.
The key factor is that the flow must be kept from separating, which will induce cavitation. This will result in wear and erosion of the bearing surfaces which will lead to bearing failure. Cavitation is prevented by the use of gently changing shapes, and the introduction of pressure at locations in the bearing where if otherwise left to itself, the flow would separate and create a low pressure cavitating region.